In recent years, a high-voltage electrical storage device having high energy density has been desired as a power supply for driving an electronic device. In particular, a lithium-ion battery and a lithium-ion capacitor are expected to be a high-voltage electrical storage device having high energy density.
A reduction in size has been desired for such a high-voltage electrical storage device having high energy density. In order to reduce the size of the electrical storage device, it is necessary to reduce the thickness of the separator that isolates the cathode and the anode in addition to the thickness of the cathode, the anode, and the like, for example. However, a short circuit may easily occur when the interval between the cathode and the anode decreases along with a reduction in size of the electrical storage device.
In particular, when the electrical storage device utilizes metal ions such as lithium ions, a dendrite tends to be produced on the surface of the electrode due to the metal ions during repeated charge and discharge. Such a dendrite normally precipitates in the form of a needle-like crystal, and is easily grown through the separator (i.e., porous film (membrane)). If a dendrite has been grown through the separator, and has reached the surface of the other electrode, the electrical storage device is short-circuited, and the charge/discharge function is lost.
It is likely that the above phenomenon occurs (i.e., reliability deteriorates) along with a reduction in thickness of the separator and a reduction in the interval between the cathode and the anode. WO2009/041395 and JP-A-2009-87562 disclose a technique that improves the battery characteristics by forming a porous layer that includes a resin binder including a polyamide, polyimide, or polyamideimide on the porous separator substrate in order to prevent the above phenomenon. JP-A-2009-54455 discloses a technique that improves the battery characteristics by forming a porous protective film that includes a binder including a fluorine-based resin and a rubber-based resin on at least one of the surface of the cathode and the surface of the anode.